The difference in physical growth between children of small and large families in Great Britain seems to affect mainly the poorer social classes. In the higher income classes boys in families with three or more children are taller at all ages than boys in small families; the reverse is true for girls. In the upper and lower manual working classes children in small families average 3-4 percent taller than those in large families at 7 and 11 years of age, and 1.4 to 2.8 percent taller at 15 years. Effects on Intelligence and Educational Performance Large numbers of children in the family diminish not only physical size but also linguistic skills, intelligence as measured by intelligence tests, and educational performance. These elements are to some extent interrelated; for example, heavier children mature earlier, and early maturers do better in school than late maturers. Experiments show that the apathy that is a major consequence of malnutrition is highly correlated with such psychological elements as lack of ambition, low self-discipline, low mental alertness, and inability to concentrate. Both physical growth and the greater cultural nurture associated with small families appear to affect intelligence. In the sample of British day school children, intelligence increased with height and decreased with family size. The average verbal reasoning scores of children over 135 centimeters tall in families of one or two children were 5 to 7.5 percent higher than those of children of the same height in families of four or more children. The difference for children of the same age but less than 135 centimeters tall between large and small families averaged about 7 percent. Tall children from both large and small families scored about 6 percent higher than short children. In studies of Scottish children the average I.Q. of only children was 113, that of children with five or more siblings was 91. In France, only children between the ages of 6 and 12 had an average mental age 1 to 2 years higher than children with eight or more siblings. The differences in educational performance between children in small and large families are especially significant when the families are separated by social class. Data from the British National Survey of Health and Development show the performance of children in families of different size in educational tests at 8 and 11 years of age. In the upper manual working class, only children and those in two-child families scored about 11 percent higher than children in families of six and about 26 percent higher than children in families of seven or more children. The difference in the lower manual working classes between only children and children in large families was about 17 percent. In the upper middle class the difference in educational performance between children in large and small families was somewhat less than 9 percent. The difference in educational performance in all classes was slightly larger at 11 years than at 8.